Type-hinting ?string for type="xs:anySimpleType" seems to be wrong

[etshy]

Describe the bug
I have a WSDL (I can't share it) that have an object with an attribute of type type="xs:anySimpleType"

It was generated with a php type hint ?string
But when that value is empty in the Response (<Element attribute="">), the SoapClient seems to generate an empty stdClass instead of an empty string or null and thus it throws an Exception.

The generator used was v4.1.12

To Reproduce
Steps to reproduce the behavior:

• During the package generation
  We used standalone=false --force

• During the usage of the generated package
  I'm using the generated classes to make the call, the generated classmap, nothing special

Expected behavior
With a type any, the property should not be typed, as it can literally be anything.

[mikaelcom]

I would way that in this casse :mixed would be more appropriate then, I was not aware of this specificity

[etshy]

I wasn't aware too and couldn't find anything "official" about that.
But I noticed that on some response payload, and found just a few StackOverflow post and it seems to work that way.